ot::Spinel::Encoder::CloseStruct() method

Closes the most recently opened struct (using `OpenStruct()`) in the current input frame. Each call to `CloseStruct()` must correspond to an earlier successfully opened struct. If a frame is ended using `EndFrame()` with remaining open structs, the `EndFrame()` method will close all the remaining structs. If no buffer space is available, this method will discard and clear the frame and return error status `OT_ERROR_NO_BUFS`.